In known transmissions of this type, the torque converter is active only during the vehicle starting phase when it up to doubles the input torque in the transmission. After start, the torque converter is locked up with the aid of the hydraulically operated friction clutch so that its input and output shafts rotate as a unit. The reason the torque converter is only utilized during starting is that it has a low efficiency compared to a toothed transmission. When the torque converter is in operation, the losses are absorbed as heat in the fluid circulating through the same. The fluid must be cooled in a heat exchanger and the more fluid which the first pump pumps through the torque converter, the lower will be the fluid temperature and the smaller the heat exchanger can be made for a given amount of heat.
Fluid from the pump with the higher pressure is directed via magnetic valves to the piston-cylinder devices, which control the lock-up clutch of the torque converter, and shift clutches in the form of multi-disc clutches. For shifting, one clutch is engaged and the other is released. A transmission of this type, a so-called “power-shift” transmission, is shown and described in SE-8700583-1 for example. High pressure and a great quantity of fluid is required during shifting, i.e. when pressing together the discs of the shift clutches, and for engaging the lock-up clutch, and this means it is necessary to dimension the high-pressure pump for the highest required pressure and flow. This means, however, that the high-pressure pump will have a much too high capacity for all other operations where fluid is only required to replace leakage, so that the pressure in the clutches will not drop. Since the shifting time during driving only amounts to a few percent of the total driving time, it will be understood that significant energy savings could be realized with another system for supplying fluid to the piston cylinder devices of the clutches.